A thermoplastic elastomer means an elastomer which is softened to have flowability upon heating and has rubber elasticity upon cooling. Specifically, at the time when an elastomer is molded, it melts at a processing temperature and it becomes possible to mold it easily by a well-known method to be used for resin molding. However, at a temperature at which it is actually used as various materials (hereinafter referred to as “use temperature”), it is an industrially extremely useful material having physical properties similar to a crosslinked rubber.
Heretofore, as thermoplastic elastomers, polymers such as block copolymers, particularly various multiblock copolymers, e.g., triblock copolymers and the like, have been known.
In general, the aforementioned block copolymer has a structure in which a “soft segment” having amorphous or rubbery physical properties and a “hard segment” that is in a crystalline or glass state at a typical use temperature of a thermoplastic elastomer. Polymer chains in the hard segment may bind to one another at the typical use temperature to exhibit properties as an elastomer. However, when it is heated at a temperature higher than the melting temperature (hereinafter also abbreviated as “Tm”) of the hard segment or the glass transition temperature (hereinafter also abbreviated as “Tg”) of the hard segment, the polymer easily exhibits a thermoplastic behavior.
The use temperature of a thermoplastic elastomer is typically at around room temperature, for example in the range of 10° C. to 40° C. but, depending on use environments and use applications, it is expected to use it at a lower temperature (e.g., 0° C. or lower) or at a higher temperature (e.g., 50° C. or higher), so that heat resistance is sometimes required. In that case, thermal properties of the hard segment become important.
As thermoplastic elastomer (TPE) compositions, styrene-based block copolymers (SBC) may be mentioned as well-known ones, and examples thereof include linear triblock copolymers such as a styrene-isoprene-styrene triclock copolymer and a styrene-butadiene-styrene triblock copolymer.
These copolymers have a well-controlled block structure and the styrene segment has relatively high Tg, so that it is known that they exhibit a relatively excellent balance between heat resistance and elastomer properties. However, since these styrene-based block copolymers are usually manufactured by successive anionic polymerization or chemical coupling of linear diblock copolymers, usable monomer species are limited. Moreover, each polymer chain requires a stoichiometric amount of a polymerization initiator and a polymerization rate is relatively low, so that the process is poor in economic efficiency.
Furthermore, since typical SBC's have a glass transition temperature of approximately 80 to 90° C., they again have flowability and are poor in heat resistance at a higher use temperature, so that use of these copolymers is limited.
In order to remedy the defects of these conventional technologies, it is particularly desired, from the viewpoints of efficiency of the process and economical efficiency of raw materials, to form these block copolymers or thermoplastic elastomer compositions by insertion or coordination polymerization of olefin monomers using a transition metal compound, so that it is investigated to improve the physical properties by olefin-based block copolymers, specifically propylene-based block copolymers.
In Non-Patent Document 1, mechanical properties of these propylene-based block copolymers are precisely evaluated. According to the evaluation, Tm of the propylene segment is high as compared to Tg of the polystyrene segment in the aforementioned styrene-based block copolymers and thereby, especially under a high temperature, the propylene-based block copolymers have higher elastomer properties as compared to the styrene-based block copolymers. However, at the manufacture of these propylene-based block copolymers, a living polymerization catalyst is used. With the living polymerization catalyst, only one polymer chain is theoretically obtained from one catalyst molecule and thus productivity is limited, so that the use applications are limited to relatively-small-amount and high-added-value fields.
Accordingly, there has been investigated a process for producing a “block-like copolymer” by a method exhibiting more excellent productivity. There has been investigated a process for forming a graft copolymer having different properties in the side chain and in the main chain by copolymerizing a polymer having a vinyl group capable of coordination polymerization at one end and a monomer, as a specific “block-like copolymer”.
Patent Document 1 discloses an olefin-based thermoplastic elastomer composition having specific physical properties, which contains a branched olefin copolymer in which a polymerizable macromonomer is copolymerized in the soft segment, and a process for manufacturing the same.
Patent Document 2 discloses a thermoplastic elastomer composition containing a branched olefin polymer having an isotactic polypropylene segment as a crystalline side chain and having an atactic polypropylene as a non-crystalline main chain, and a process for manufacturing the same.
Patent Document 3 discloses a composition containing a branched propylene copolymer having an isotactic polypropylene segment in the side chain and having a propylene/ethylene copolymer in the main chain.